1. Field of the Invention
The present invention relates to a variable resistor ("varistor"), particularly to a chip type varistor and ceramic compositions for the same.
2. Description of Related Art
A varistor is a circuit element whose resistance abruptly decreases when the voltage applied to the element exceeds a predetermined level. On the other hand, the resistance is extremely large when the applied voltage is lower than that level. Because of such characteristics, varistors are used for protecting semiconductor elements from a surge voltage, for example.
Varistors made of various materials and having various structures are known. For example, varistors with lead wire are made from SiC system, ZnO system, SrTiO.sub.3 system or TiO.sub.2 system materials. Some chip type varistors include ZnO or SrTiO.sub.3 as main components.
The current trend toward smaller circuits for high frequencies necessitates electronic components that accommodate smaller sizes and higher frequencies, and the trend toward lower circuit driving voltages necessitates components that operate on low voltages.
Therefore, it is preferable that varistors have a small electrostatic capacity to be compatible with the high frequencies and to be usable to absorb noises in signal circuits and the like, and the voltages of such varistors must be limited to small values in order for them to operate low voltages.
In the case of ZnO type varistors, however, the area of the electrodes thereof must be significantly reduced to obtain a small electrostatic capacity and a varistor voltage of a few volts since they have high apparent dielectric constants that are as high as several hundreds. However, this results in a reduction in surge resistance at the same time.
In the case of the SrTiO.sub.3 type and TiO.sub.2 type varistors, the apparent dielectric constants are higher than those of ZnO type varistors and can be on the order of several thousands or several tens of thousands, but it is more difficult to obtain a small electrostatic capacity and a varistor voltage of a few volts.
Meanwhile, it is easier to obtain SiC type varistors having a small capacity because they have low apparent dielectric constants. However, they have a voltage non-linearity coefficient a smaller than those of other types of varistors. For example, the constant .alpha. is only about 7 for SiC type varistors while it is several tens for ZnO type resistors.